1. Field of the Invention
The invention relates to truck controls and in particular to safety controls for preventing inadvertent backing of a dump truck while raising the dump body.
2. Description of the Prior Art
In one conventional dump truck, the truck drive includes a hydraulically controlled power shift transmission including a plurality of automatic forward speeds and one reverse speed selection. A manual selector valve for conditioning the transmission is located at the transmission for operation by a solenoid operated shift cylinder. The shift cylinder is controlled by an electrical switch means responsive to a console shift lever under the manual control of the operator in the truck cab.
The console shift lever may be selectively positioned in a reverse position, a neutral position, or in any one of the plurality of different forward positions, to place the transmission in the desired operating range.
The dump body of such dump trucks is positioned by a hydraulic hoist which is selectively operated by a hydraulic control provided with a manual control lever. The hydrualic hoist control may be selectively positioned in a "hold" position for maintaining the dump body in a preselected position, a "float" position permitting the dump body to move downwardly under the weight thereof from an elevated position to a lowered position, and "raise" and "down" positions for powering the dump body selectively to the raised or lowered positions.
A problem arises in the use of such dump trucks in that operation of the hoist means in the power driven mode, i.e., in the "raise" or power "down" modes, requires an increased flow of hydraulic fluid from the engine driven pump. The operator customarily accelerates the engine to at least a high idle position to provide this necessary additional hydraulic pumping action. In the event the operator forgets to shift the truck from the reverse position in initiating the power dumping action, the extremely hazardous condition arises of causing the truck to move rearwardly inadvertently. This condition is extremely dangerous where the truck is used to dump into a pit or over an embankment.
A number of different controls for neutralizing a vehicle transmission under certain operating conditions have been developed in the art. Illustratively, as shown in the Edward J. Lammers et al United States Letters Patent 3,565,220, owned by the assignee hereof, a control system is provided having an interconnection between the brakes and the transmission to neutralize the transmission when either one of two different sets of brakes is applied.
In United States Letters Patent 3,033,333 of Ulysses A. Breting et al, a cutoff valve is provided for placing the transmission in a neutral condition upon application of the vehicle brakes. Similarly, in United States Letters Patent 3,181,667 of Kenneth R. Lohbauer et al and United States Letters Patent 3,339,672 of Richard E. Crandall, controls are provided for neutralizing the vehicle transmission by operation of the vehicle brakes. In the Crandall structure, the control further neutralizes the transmission selectively by relieving clutch fluid pressure.